Thin organic layers having nonlinear optical properties and a surface area measuring several square centimeters offer a number of advantages. Such layers have many applications, particularly in the area of photorefringence, for uses such as the optical storage of data, logical functions and dynamic interconnections.
Polydiacetylene polymers having the general formula [R--C--C.dbd.C--C--R'] are known for having significant nonlinear susceptibilities of the third order (x.sup.(3)). Of these polymers, monocrystalline polydiacetylene exhibits the highest values of x.sup.(3).
Previously known methods for preparing polydiacetylene crystals, however, have not resulted in the formation of products which are very useful. Although some known methods do result in the formulation of good quality polydiacetylene monocrystals, the dimensions of these products are relatively small, usually being less than 1 cm.sup.2 in size. Moreover, the stresses produced within the crystal during polymerization can cause a number of defects within the product, even leading in some cases to the destruction of the crystal.
The formation of fine polycrystalline diacetyl layers permits avoidance of the problems described above caused by modification of the geometric parameters of the crystalline lattice during polymerization. With this method, however, it is generally possible to maintain the crystalline order only on the scale of the microcrystal. The arrangement of the microcrystals is most often random and thus the macroscopic value of the cubic capacity of such crystals is averaged over all the possible orientations.
The process of "epitaxy" permits an oriented growth of a substance by depositing it upon a monocrystalline substrate. This process has been utilized with polydiacetylenes in a variety of applications. Mineral monocrystals such as KCl or KBr are normally utilized as supports, leading in general to the formation of bioriented polycrystalline layers whose nonlinear optical properties are difficult to use practically.
Thus, at present, the methods utilized to form polydiacetylene crystals tend to result either in small, fragile monocrystals or in polycrystalline layers which, because they lack a single orientation, do not provide the optical performance otherwise expected from crystalline polydiacetylenes. The process of the present invention, in contrast, produces a product which reconciles these two requirements, i.e., the large, non-linear capacities of the mono-oriented layers and the large optical surface quality obtainable with microcrystalline films.
Monocrystalline polydiacetylene layers have previously been obtained by methods such as those disclosed in U.S. Pat. No. 4,684,434 to Thakur et al. The method described by this reference is carried out in two stages, the first involving the creation of a liquid layer wherein the monomer is either in the molten state or in solution, following which the liquid layer is placed between two surfaces comprised of glass, quartz or monocrystals of mineral salts and subjected to an elevated pressure. In the second stage of the process, the monomer solidifies, either due to the slow evaporation of the solvent or by slow cooling. The monomer is subsequently polymerized by exposure to ultraviolent radiation and the monocrystals thus obtained can reach several tenths of a square centimeter in surface area.
An alternate method for the formation of monocrystaline polydiacetylene layers is described in Applied Physics Letters, 51 (23), 1957 (1987). The process disclosed by this reference makes it possible to obtain polycrystalline layers having a surface area of several square centimeters, with good alignment of the crystals relative to one another. The process comprises a first step of melting the purified monomer in a quartz crucible, heating the melted monomer to 125.degree. C. under a vacuum of 8-10.sup.-4 Pascals and collecting the condensate upon a glass substrate maintained at a temperature of 10.degree. C. This produces a monomer film, having a thickness of several tens of nanometers. In a second stage, the film is polymerized in air by ultraviolet radiation, resulting in the formation of an isotropic film.
The crystals are oriented in a third step, during which the polycrystalline layer is rubbed lightly in one direction with a silicone cloth. The layer is thus thinned and the residual crystals are oriented at the surface of the glass substrate. The steps described above may be repeated several times in order to form a film several hundred nanometers thick, comprising polycrystals whose face (010) is parallel to the substrate and whose direction [001] is aligned with the direction of mechanical friction imparted by wiping the polycrystalline layer with the silicon cloth. However, the above-described process is known to result in cracks extending perpendicular to the direction of friction that introduce discontinuities into the coating layer.
With the use of a third method, described for example in Journal of Chemical Physics, 88 (10), 6647 (1988), the crystal orientation is obtained by epitaxy, i.e., a polycrystalline layer is deposited upon a mineral monocrystal on which the small unit crystals are oriented. In the reference described above, monomer crystals of [1,6-di-(n-carbazolyl)-2,4-hexadiyne] are heated in a crucible under a vacuum of 10.sup.-5 Pa. A freshly cloven monocrystal, e.g., KBR, on which the monomer is to be deposited, is placed at a distance from the crucible. A uniform film is obtained, which is then heated to a temperature of 150.degree. C. to complete the polymerization reaction.
A variant of this method utilizes an organic monocrystal of 2-5 piperazine-dione. Continuous films may thus be obtained having a thickness of 25 nm or more with a large surface area and comprising bioriented polycrystals. The diffraction spots are split and the corresponding angle is at least 10.degree..
Thus it may be seen that the first of the three techniques described above results in the formation of a layer comprising small monocrystals, whereas the third technique provides an oriented polycrystalline layer which is bioriented. The second technique is exceedingly laborious and difficult to use in industrial processes. The application of mechanical friction, i.e., by rubbing with a silicon cloth, leads to the formation of cracks within the final coating.